1. Field of the Invention
The present invention relates to communication apparatuses such as a portable radio apparatus capable of transmitting and receiving an audio signal, and more particularly to a communication apparatus in which a sound level of a speaker can be automatically adjusted according to a surrounding noise.
2. Prior Art
Recently, a portable telephone set has been widespread with progressed mobile radio communication, and people have come to easily talk over the telephone at any place as well as specific places such as a telephone booth, etc. This is an important feature of the mobile radio telephone system. However, a noise problem arises in cases where the telephone is used in noisy surroundings.
In Japanese Patent Laid-open Publication No. HEI 4-82331 (publication date: Mar. 16, 1992), a portable telephone set is provided with a microphone for detecting a surrounding noise. The surrounding noise level detected by the microphone is compared with a reference level, and based on a result of the comparison, a sound volume is controlled according to the surrounding noise level so that a receiving speech can easily be heard.
In another telephone set disclosed in Japanese Patent Laid-Open Publication No. HEI 2-250455 (publication date: Oct. 8, 1990), a telephone transmitter is used to detect a surrounding noise, and the sound volume of a telephone receiver is controlled according to the surrounding noise level. In this telephone set, the surrounding noise level is detected from a transmitting signal including a transmission speech signal and a surrounding noise. Therefore, a transmitting signal is recognized as a surrounding noise when the telephone set is in its on-hook state where there is no transmission speech, and the sound volume is regulated according to the surrounding noise.
However, in the portable telephone set disclosed in the former publication (HEI 4-82331), since the noise detecting microphone is needed in addition to a telephone transmitter and a telephone receiver, the number of parts is increased and an extra space for providing the noise detecting microphone is needed. Therefore, it is not preferable that such a microphone used only to detect a surrounding noise is provided in a portable telephone set which is required to be reduced in size.
In the other telephone set disclosed in the latter publication (HEI 2-250455), the telephone transmitter is used to detect a surrounding noise, so the telephone set can be applied to a small-size communication device. However, since the surrounding noise is detected at the on-hook state and the sound volume is regulated based on the level of this detected noise, the sound volume cannot be regulated according to the surrounding noise at the off-hook state when taking is actually performed. Therefore, even if the level of the surrounding noise changes during talking, the sound volume is fixed at a certain level which was set while on-hook, causing a problem that the receiving speech sound is difficultly heard in noisy surroundings.